A strategy for controlling the central torsion angle in biphenyl-based molecular-scale bridges

The synthesis of a series of crown ether ligands (C4-C6) based upon the 2,2'-biphenyl unit is described. In the series, the length of the polyether chain connecting the two oxygen atoms increases progressively. These polyethers are moderately fluorescent in acetonitrile solution, but this emission is partially quenched upon the addition of various alkali metal cations. Spectrofluorimetric titrations performed with MClO4 (M = Li+, Na+) or KPF6 show that binding to crown ethers C4-C6 involves the formation of a 1:1 cation/ligand complex. The geometry of the resultant complex depends on the nature of the bound cation and also on ionic strength. In particular, the photophysical properties of the polyether are sensitive to the torsion angle around the central connector in the biphenyl group. This approach, therefore, could be used to systematically vary the dihedral angle for biphenyl-based molecular-scale bridges.